The present invention relates to a method of transcoding input coded signals previously quantized with a first quantization scale Q1 into output coded signals quantized with a second quantization scale Q2.
Digital video compression techniques (such as MPEG-2), originally designed to broadcast digital video in the most cost-effective way, are now spreading in studios and in consumer products, since they allow to reduce transmission and storage costs. As a consequence, uncompressed video links are progressively replaced by compressed video links. This evolution is illustrated in FIG. 1:
at the upper part of said figure, an uncompressed video bitstream UVB is processed by an encoder COD, whose output coded bitstream is received by a transmission channel or recording medium CM, having a bitrate constraint of X bits/sec, and later decoded by a decoder DECOD for reconstructing and displaying pictures corresponding to the original ones;
at the lower part, the original bitstream UVB is coded in an encoder COD yielding a bitrate of Z bits/sec(Z greater than X), and the bitstream thus compressed is sent towards a transcoder TRANS whose output coded bitstream at Y bits/sec is also received by a transmission channel or recording medium CM and later decoded.
However, MPEG video systems are not optimized for successive encoding operations: for the same picture quality, direct encoding of uncompressed video requires a lower number of bits than transcoding of compressed video. Moreover, the straightforward transcoding solution, that consists of using a decoder and an encoder, is very expensive due to the encoder part.
A bitrate transcoder that is not based on cascaded decoder and encoder has been already proposed in the European patent application filed on Nov. 17, 1999, with the number 99402854.6 (PHF99608). Such a system, based on a re-quantization of the discrete cosine transform (DCT) data, is very simple, but only correct for intra-coded pictures. The re-quantization of inter pictures does not take into account the re-quantization carried out on the reference pictures. Since the predicted pictures may later be used themselves as reference pictures, errors may add up to the extent that a drift begins to be noticeable in the final decoded video sequence corresponding to the transcoded stream. Said drift impairs more and more the performance of the transcoder as the bitrate change due to the transcoder increases.
It is therefore the object of the invention to propose a new type of transcoder with reduced bitrate losses.
To this end the invention relates to a method of transcoding input coded signals previously quantized with a first quantization scale Q1 into output coded signals quantized with a second quantization scale Q2, comprising at least the steps of:
a) decoding the input coded signals, resulting in input decoded signals;
b) modifying the quantization step of said decoded signals;
c) encoding the modified decoded signals for obtaining the output coded signals;
wherein said modifying step comprises the following sub-steps:
(i) between said decoding and encoding operations, selecting one out of two parallel requantizing branches according to the picture coding type of said input decoded signals;
(ii) when said coding type corresponds to pictures used as references for future pictures to be transcoded, requantizing said input decoded signals by means of successive sub-operations including an inverse quantization with respect to the first quantization step Q1, a prediction, and a requantization with the second quantization step Q2;
(iii) when said coding type does not correspond to reference pictures, requantizing said input decoded signals by means of a filtering sub-operation the transfer function of which is ((Q1/Q2)+xcex5) where xcex5 is a correction factor.